Method for preventing unnecessary retransmission due to delayed transmission in wireless network and communication device using the same

ABSTRACT

A method for reducing unnecessary retransmissions due to transmission delays in a wireless network environment, comprising: measuring a packet inter-arrival time with respect to a received data packet; transmitting an acknowledgement packet corresponding to the data packet received at the packet inter-arrival time in excess of a first threshold time when the measured packet inter-arrival time exceeds the first threshold time; and suspending transmission of an acknowledgement packet corresponding to a new data packet when a data packet received after having transmitted the acknowledgement packet is the new data packet that has not previously been received.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority from Korean PatentApplication No. 10-2005-0012802 filed on Feb. 16, 2005 in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Devices and methods consistent with the present invention relate totransmission control in a transmission control protocol (TCP). Moreparticularly, the present invention relates to a method for preventingunnecessary retransmission caused due to delayed transmission in awireless network environment, and a communication device using the same.

2. Description of the Related Art

The transmission control protocol (TCP) is used together with theInternet protocol (IP) for data transmission between communicationdevices. A main function of TCP is to stabilize the transmission of datathrough a network. For this, TCP uses a congestion control mechanism.The TCP congestion control mechanism controls the data packettransmission by observing network congestion while the data packets arebeing transmitted, and retransmits any data packet(s) lost due tonetwork congestion. The TCP congestion control mechanism according tothe conventional art will be described with reference to FIGS. 1 and 2.

FIG. 1 illustrates changes in the transmission rate according to theconventional slow-start/congestion avoidance algorithm.

The TCP of a communication device that transmits data packets(hereinafter referred to as a “transmitting device”) adjusts the size ofa congestion window (hereinafter abbreviated as “CWND”), and transmitsone or more data packets simultaneously according to the size of CWND.As the CWND size increases, the number of data packets available forsimultaneous transmission increases. The initial CWND size (CWND 1) isone TCP segment.

A communication device that receives a data packet (hereinafter referredto as a “receiving device”) transmits an acknowledgement packet (ACKpacket) to the transmitting device. The TCP of the transmitting deviceincreases the CWND size by one TCP segment whenever it receives anacknowledgement packet transmitted by the receiving device. Accordingly,the CWND size increases exponentially, which is called a slow-startalgorithm. Changes in the CWND size according to the slow-startalgorithm are depicted in the slow-start period 110.

When the CWND size approaches the slow-start threshold (hereinafterabbreviated as “SSTHRESH”), that is, SSTHRESH 1 while the slow-startalgorithm is in progress, the TCP of the transmitting device uses acongestion avoidance algorithm that linearly increases the CWND sizeconsidering the network congestion. In the congestion avoidancealgorithm, the TCP increases the CWND size by 1/CWND whenever itreceives an acknowledgement packet. Changes in the CWND size accordingto the congestion avoidance algorithm are depicted in the congestionavoidance period 120.

When no acknowledgement packet corresponding to the transmitted datapacket is received, the TCP of the transmitting device waits until theacknowledgement packet is received, without increasing the CWND size.However, if no acknowledgement packet is received after a predeterminedtime period, the TCP generates a retransmission timeout 130. The timeperiod from when the TCP transmits a data packet to when it generates aretransmission timeout is about two times the round trip time (RTT) of apacket.

Once the retransmission timeout is generated, the TCP decides that thetransmitted data packet has been lost due to network congestion. Then,the TCP sets the CWND size to one TCP segment so that the amount of datato be transmitted is reduced. In addition, the TCP sets the slow-startthreshold (SSTHRESH 2) to half the CWND size (CWND 2) immediately beforethe retransmission timeout is generated. According to the slow-startalgorithm, the TCP resumes transmitting the lost data packets (the datapackets whose acknowledgement packets have not been received).

FIG. 2 illustrates changes in transmission rate according to acongestion control algorithm which employs the conventional fastretransmission and fast recovery algorithm.

In the slow-start/congestion avoidance algorithm, loss of data packetsin the course of transmission is decided based only on whether theretransmission timeout is generated. However in the fast retransmissionalgorithm, if a predetermined number of duplicate acknowledgementpackets (duplicate ACKs) has been received even before theretransmission timeout is generated, it is decided that the data packetsare lost.

A duplicate acknowledgement packet is generated by the TCP of thereceiving device when data packets transmitted by the transmittingdevice and those received by the receiving device are different insequence. The sequence of data packets can be ascertained through thesequence numbers set in the headers of the data packets.

When an acknowledgement packet to a specific data packet has not beenreceived while the transmitting device is using to theslow-start/congestion avoidance algorithm, the TCP waits for theacknowledgement packet until a retransmission timeout is generated. Whena predetermined number of duplicate response packets have been receivedbefore the retransmission timeout is generated, the TCP considers thedata packets lost though no retransmission timeout has been generated.At this time, the TCP uses the fast retransmission algorithm toretransmit the data packets considered as having been lost. Changes inthe transmission rate due to the fast retransmission algorithm aredepicted in the fast retransmission period 220.

Usually the fast retransmission algorithm is applied when threeduplicate acknowledgement packets have been received. When only one ortwo duplicate acknowledgement packets are received, it may be decidedthat the data packets have not arrived sequentially as transmitted bythe transmitting device since they respectively suffered from delayedtransmissions in the network while they were transmitted to thereceiving device, rather than considering the data packets lost.

When the transmitting device uses the fast retransmission algorithm, theCWND size is set to a value$\left( {\frac{{CWND}\quad 3}{2} + 3} \right)$that is half the CWND size (CWND 3) immediately before the duplicateacknowledgement packets are received plus three TCP segments.

In the fast recovery algorithm, when an acknowledgement packet to theretransmitted data packet according to the fast retransmission algorithmis received, the TCP allows the transmitting device to immediately usethe congestion avoidance algorithm without having to use the low-startalgorithm. For this, a SSTHRESH value and a CWND size are set to halfthe CWND size (CWND 3) immediately before the fast retransmissionalgorithm is applied. Changes in transmission rates according to thefast recovery algorithm are depicted in the fast recovery period 230.

When a retransmission timeout is generated under the condition that anacknowledgement packet to a specific data packet has not been received,the TCP employs the slow-start/congestion avoidance algorithm. When apredetermined number of duplicate acknowledgement packets have beenreceived before a retransmission timeout is generated, and under thecondition that an acknowledgement packet to a specific data packet hasnot been received, the TCP employs the fast retransmission and fastrecovery algorithm.

In the wireless network, transmission of data packets may abruptly andtemporarily be delayed due to a change in the wireless environment,which is called “delay spike.” A delay spike may be caused throughmobility of a communication device, retransmission of a link layer tocompensate for loss generated due to fading of a communication device,and periodical searches of a channel to search for a mobile terminatecall.

Once a delay spike is generated, the transmitting device may not receivean acknowledgement packet to a transmitted data packet until aretransmission timeout is generated. In this case, the transmittingdevice decides that the data packets that it transmitted have been lost,and retransmits the data packets. However, when a data packet issuccessfully received by the receiving device despite delayedtransmission due to the delay spike, retransmission of the data packetby the transmitting device will result in wasting resources of awireless link. This problem according to the conventional art will bespecifically described with reference to FIG. 3.

FIG. 3 illustrates packet transmission between communication devicesaccording to the conventional art. In this figure, “N” to “N+9” refersto the sequence numbers of data packets.

Referring to this figure, the receiving device that has received a datapacket “N” transmits a response packet thereto, and the transmittingdevice consecutively transmits data packets “N+1” to “N+7.” However, asillustrated, data packets “N+1” to “N+7” transmitted from thetransmitting device suffer from delayed transmissions 310. Delayed datapackets may be stored in a queue of a relay device that relays packetsbetween the transmitting device and the receiving device.

Because of this delayed transmission, a retransmission timeout 320 isgenerated in the transmitting device before it receives acknowledgementpackets in response to the transmitted data packets “N+1” to “N+7.”

Because the retransmission timeout 320 has been generated, thetransmitting device decides that the data packets “N+1” to “N+7” arelost and attempts retransmission thereof. At this time, because theretransmission timeout has been generated the TCP of the transmittingdevice sets the CWND size to 1 TCP segment, and thus, only the firstdata packet “N+1” is preferentially retransmitted.

When the transmission delay is released 330 within a predetermined timedue to a change in the wireless environment, the data packets “N+1” to“N+7” are successfully transmitted to the receiving device and thereceiving device transmits an acknowledgement packet to each data packet340. Accordingly, the retransmission timeout 320 generated in thetransmitting device becomes a spurious timeout due to the delay spike.

As the acknowledgement packets from the receiving device are received,the transmitting device adjusts the CWND size and transmits data packetsfollowing the data packet “N+1.” When all of the data packets “N+1” to“N+7” are retransmitted, new data packets “N+8,” “N+9” and so on aretransmitted.

However, retransmission by the transmitting device of the transmitteddata packets “N+1” to “N+7” will result in wasting resources of thewireless link since the data packets were merely delayed but weresuccessfully transmitted to the receiving device. In other words,retransmission of the data packets “N+1” to “N+7” is the unnecessaryretransmission.

At this time, the retransmitted data packets “N+1” to “N+7” are onstandby in a queue of a relay device that relays packets between thetransmitting device and the receiving device while data packets “N+1” to“N+7” is being transmitted, which were previously transmitted. For thisreason, the retransmitted data packets may be a little delayed intransmission as depicted in FIG. 3.

The receiving device that received the retransmitted data packets “N+1”to “N+7” again receives the same data packets, and it generatesduplicate acknowledgement packets 350. When more than a predeterminednumber of duplicate response packets are transmitted to the transmittingdevice, the transmitting device operates according to the fastretransmission algorithm as described with reference to FIG. 2, and theCWND size decreases accordingly. However, at this time the fastretransmission algorithm is operated because of the unnecessaryretransmission, and thus, reduction of the CWND size results inunnecessarily reducing the data transmission rate, thereby greatlywasting the wireless link resources.

In this case, the transmitting device retransmits the alreadytransmitted data packets in response to the duplicate response packets.This retransmission is referred to as a “spurious retransmission,” andthis spurious retransmission wastes the wireless link resources. In theillustrated example, the already transmitted packet, N+8, isretransmitted because of the duplicate acknowledgement packet to thedata packet, N+7 360.

According to the conventional art, even though a data packet issuccessfully transmitted after the transmission thereof is delayed, thetransmitting device decides that the data packet was lost, and thus,retransmits the concerned data packet. This retransmission causes theproblem that wireless link resources are wasted. In addition, theretransmitted data packets cause the receiving device to generateduplicate response packets, thereby resulting in extra reduction of thetransmission rate by the transmitting device (unnecessary fastretransmission). In addition, the wireless link resources are furtherwasted because of the spurious retransmission generated due to theduplicate response packets.

SUMMARY OF THE INVENTION

The present invention provides a method and device which may decreaseunnecessary retransmission of data packets due to delayed transmissionin the wireless network environment.

According to an aspect of the present invention, there is provided amethod for reducing unnecessary retransmissions due to transmissiondelays in a wireless network environment, comprising: measuring a packetinter-arrival time with respect to a received data packet; transmittingan acknowledgement packet corresponding to the data packet received atthe packet inter-arrival time in excess of a first threshold time whenthe measured packet inter-arrival time exceeds the first threshold time;and suspending transmission of an acknowledgement packet correspondingto a new data packet when a data packet received after havingtransmitted the acknowledgement packet is a new data packet that has notpreviously been received.

According to an aspect of the present invention, there is provided acommunication device, comprising: an inter-arrival time measuring unitwhich measures a packet inter-arrival time of a received data packet; acalculation unit which calculates a first threshold time using thepacket inter-arrival time measured by the inter-arrival time measuringunit; a determination unit to compare the packet inter-arrival timemeasured by the inter-arrival time measuring unit with the firstthreshold time calculated by the calculation unit; and a control unitwhich controls transmission of an acknowledgement packet correspondingto a data packet received at a packet inter-arrival time exceeding thefirst threshold time when the determination unit determines that thepacket inter-arrival time exceeds the first threshold time, and suspendstransmission of an acknowledgement packet corresponding to a new datapacket when the new data packet received after having transmitted theacknowledgement packet has not been previously received.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the attached drawings in which:

FIG. 1 illustrates changes in transmission rates according to theconventional slow-start/congestion avoidance algorithm;

FIG. 2 illustrates changes in transmission rates according to thecongestion control mechanism which applies the conventional fastretransmission and fast recovery algorithm;

FIG. 3 illustrates transmission of packets between communication devicesaccording to the conventional art;

FIG. 4 is a block diagram illustrating a communication device accordingto an exemplary embodiment of the present invention;

FIG. 5 is a flow chart illustrating a process of operating acommunication device according to an exemplary embodiment of the presentinvention;

FIG. 6 is a flow chart illustrating a method of preventing unnecessaryretransmission in a wireless network environment according to anexemplary embodiment of the present invention;

FIG. 7 is a flow chart illustrating a method of preventing spuriousretransmission by a transmitting device according to an exemplaryembodiment of the present invention;

FIG. 8 illustrates transmission of packets between communication devicesaccording to an exemplary embodiment of the present invention;

FIG. 9 illustrates transmission of packets between communication devicesaccording to another exemplary embodiment of the present invention;

FIG. 10 illustrates transmission of packets between communicationdevices according to a further exemplary embodiment of the presentinvention; and

FIG. 11 illustrates transmission of packets between communicationdevices according to a still further exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.Advantages and features of the present invention and methods ofaccomplishing the same may be understood more readily by reference tothe following detailed description of exemplary embodiments and theaccompanying drawings. The present invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete and will fullyconvey the concept of the invention to those skilled in the art, and thepresent invention will only be defined by the appended claims. Likereference numerals refer to like elements throughout the specification.

Exemplary embodiments of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 4 is a block diagram illustrating a communication device accordingto an exemplary embodiment of the present invention.

The communication device comprises a packet receiving unit 410, an errordetecting unit 415, a data extracting unit 420 and a data transferringunit 425. The communication device further comprises an inter-arrivaltime measuring unit 430, a determination unit 435, a calculation unit440, a control unit 445, a acknowledgement packet generating unit 450and a packet transmitting unit 455.

The packet receiving unit 410 receives a data packet transmitted fromanother communication device through a wireless medium.

The error detecting unit 415 determines whether the data packet receivedby the packet receiving unit 410 has an error. This error determinationmay be made through a checksum field included in a TCP header of thedata packet.

When there is no error in the received data packet, the data extractingunit 420 extracts data from the data packet, and the extracted data istransferred to an application through the data transferring unit 425.

The inter-arrival time measuring unit 430 measures the packetinter-arrival time. The packet inter-arrival time refers to the timeinterval between the arrival time of the previously received data packetand the arrival time of the currently received data packet. Accordingly,the inter-arrival time measuring unit 430 can measure a packetinter-arrival time whenever the packet receiving unit 410 receives adata packet.

The determination unit 435 compares the packet inter-arrival timemeasured by the inter-arrival time measuring unit 430 with a firstthreshold time. The first threshold time refers to a packet round-triptime of the transmitting device that can be estimated by the receivingdevice. When the packet inter-arrival time is in excess of the firstthreshold time, the determination unit 435 may determine that atransmission delay has been caused in the current wireless network.Preferably, the first threshold time is the upper limit value of a firsttime zone (to be described later).

When the packet inter-arrival time is less than the first thresholdtime, the determination unit 435 can determine whether the packetinter-arrival time measured by the inter-arrival time measuring unit 430belongs to either the first time zone or a second time zone.

For example, when the packet inter-arrival time measured by theinter-arrival time measuring unit 430 is less than the upper limit valueof the second time zone, the determination unit 435 may determine thatthe packet inter-arrival time falls under the second time zone. However,when the packet inter-arrival time is in excess of the upper limit valueof the second time zone, the determination unit 435 may determine thatthe packet inter-arrival time falls under the first time zone.

Hereinafter, the first time zone and the second time zone will bedescribed.

As described above with reference to the conventional art, the TCPdetermines a data transmission rate by adjusting the CWND size, andaccordingly, the transmitting device can transmit a series of datapackets at the same time. The second time zone refers to the time periodwithin which packet inter-arrival times measured with respect to thereceived data packets can be distributed, when a series of data packetssimultaneously transmitted by the transmitting device are received bythe packet receiving unit 410. The receiving device can determine thatdata packets received at the packet inter-arrival times falling underthe second time zone have been transmitted at the same time by thetransmitting device.

Once an acknowledgement packet is received, the TCP of the transmittingdevice increases the CWND size so as to raise the transmission rate.Here, the time interval between the time when the transmitting devicetransmits a series of data packets according to the CWND size and thetime when it transmits a next series of data packets according to thenewly set CWND size after having received acknowledgement packets fromthe receiving device has a value approximately equal to the packetround-trip time. To interpret this in terms of the receiving device, thetime interval between the time when the receiving device receives thelast data packet among the series of data packets received at the packetinter-arrival time falling under the second time zone, and the time whenit receives the first data packet among the next series of data packetsreceived at the packet inter-arrival time falling under the second timezone after having transmitted the acknowledgement packets has a valueapproximately equal to the packet round-trip time. The first time zonerefers to a time period within which the packet inter-arrival times thatcan be estimated as the packet round-trip time of the transmittingdevice are distributed. That is, it may be understood that the firsttime zone is a period of time within which times from when the receivingdevice transmits an acknowledgement packet to when the receiving devicereceives a data packet newly transmitted by the transmitting device thatreceived the acknowledgement packet can be distributed. Accordingly,when the packet inter-arrival time exceeds the upper limit value (firstthreshold time) of the first time zone, it is determined that a delay intransmission has been generated in the wireless network.

The packet inter-arrival times that can be included in the first timezone and the second time zone will be described, by way of example, withrespect to the first to third data packets transmitted by thetransmitting device.

The transmitting device sets a first CWND size to 1 TCP segment when ittransmits a data packet, and accordingly, one data packet can betransmitted. Upon receiving an acknowledgement packet corresponding tothe first data packet from the receiving device after having transmittedthe first data packet, the transmitting device increases the CWND sizeto two times, and accordingly, the transmitting device can transmit twodata packets at the same time; that is, the second data packet and thethird data packet are transmitted at the same time.

Once a data packet transmitted by the transmitting device is received,the inter-arrival time measuring unit 430 of the receiving devicedetermines a packet inter-arrival time. The packet inter-arrival timemeasured relative to the second data packet (the time interval betweenarrival of the first data packet and arrival of the second data packet)is within the first time zone. The packet inter-arrival time measuredrelative to the third data packet (the time interval between arrival ofthe second data packet and arrival of the third data packet) is withinthe second time zone.

The calculation unit 440 calculates the upper limit value of the firsttime zone and that of the second time zone. As a result of adetermination by the determination unit 435, when the packetinter-arrival time is within the first time zone, the calculation unit440 newly calculates the upper limit value of the first time zone. As aresult of a determination by the determination unit 435, when the packetinter-arrival time is within the second time zone, the calculation unit440 newly calculates the upper limit value of the second time zone.Accordingly, the first time zone and the second time zone can be set bythe calculation unit 440.

According to an exemplary embodiment of the present invention, the firsttime zone can be calculated through Equation 1 as follows.First time zone=(T _(s) +V _(s) , T _(L) +V _(L))   [Equation 1]

In this equation, “T_(L)” refers to the mean value of packetinter-arrival times of the first time zone, “V_(L)” to the deviation ofthe mean value, “T_(L)”. “T_(S)+V_(S)” refers to the upper limit valueof the second time zone (to be described later).

According to an exemplary embodiment of the present invention, the meanvalue, “T_(L)” can be calculated through Equation 2 as follows.T _(L) =a*T _(L−1) +b*t _(L)   [Equation 2]

In this equation, “T_(L−1)” refers to the previously calculated meanvalue of packet inter-arrival times included in the first time zone,“t_(L)” refers to a concerned packet inter-arrival time when the packetinter-arrival time measured relative to the currently received datapacket is included in the first time zone. In addition, “a” refers tothe weight of “T_(L−1)” and “b” refers to the weight of “t_(L)”, and “a”and “b” follow the relation: a+b=1. It is preferable that the weight “a”be larger value than the weight “b.” Through this equation, rapidchanges of the mean value “T_(L)” due to the newly measured packetinter-arrival time can be prevented, thereby obtaining a stable meanvalue.

In Equation 2, the initial value of the mean value “T_(L)” may be set tothe packet inter-arrival time measured relative to the second datapacket received by the packet receiving unit 410, among the data packetstransmitted from the transmitting device.

According to an exemplary embodiment of the present invention, thedeviation “V_(L)” of the mean value “T_(L)” can be calculated throughEquation 3 as follows.V _(L) =c*V _(L−1) +d*|V _(L−1) −T _(L)|  [Equation 3]

In this Equation 3, “V_(L−1)” refers to a deviation calculated prior toobtaining the deviation “V_(L)”, and “T_(L)” is a mean value calculatedthrough Equation 2.

In addition, “c” refers to the weight of “V_(L−1)” and “d” to the weightof “T_(L),” and “c” and “d” follow the relation: c+d=1. It is preferablethat the weight “c” be larger than the weight “d.” Through thisequation, rapid changes of the deviation due to the mean value can beprevented, thereby obtaining a stable variation.

The initial value of the deviation “V_(L)” may be set to a real numbertimes the initial value of the mean value “T_(L)”, and it is preferablefor this real number to be between 0 and 1; more preferably, the realnumber may be 0.1. However, this is merely for illustrative purposes,and the initial value of the deviation “V_(L)” may vary according to thewireless network environment.

The calculation unit 440 may set the initial value of the upper limitvalue of the first time zone using the initial value of the mean valueT_(L) and the initial value of the variation “V_(L).” Accordingly, whenthe determination unit 435 determines that the packet inter-arrival timemeasured relative to the newly received data packet is included in thefirst time zone, the calculation unit 440 newly sets the first timezone, reflecting the packet inter-arrival time “t_(L)” measured relativeto the newly received data packet.

According to an exemplary embodiment of the present invention, thesecond time zone can be calculated through Equation 4 as follows.Second time zone=(0, T _(S) +V _(S))   [Equation 4]

In Equation 4, “T_(S)” refers to the mean value of packet inter-arrivaltimes included in the second time zone, and “V_(S)” refers to thedeviation of the mean value “T_(S).” The mean value “T_(S)” can becalculated through Equation 5 as follows.T _(S) =e*T _(S−1) +f*t _(S)   [Equation 5]

In Equation 5, “T_(S−1)” refers to the previously calculated mean valueof packet inter-arrival times included in the second time zone, “t_(S)”refers to a concerned packet inter-arrival time when the packetinter-arrival time measured relative to the currently received datapacket is included in the second time zone.

In addition, “e” refers to the weight of “T_(S−1)” and “f” refers to theweight of “t_(S),” and “e” and “f” follow the relation: “e+f=1.” It ispreferable that the weight “e” be larger than the weight “f.” Throughthis equation, rapid changes of the mean value “T_(S)” due to the newlymeasured packet inter-arrival time can be prevented, thereby obtaining astable mean value.

In Equation 5, the initial value of the mean value “T_(S)” may be set tothe packet inter-arrival time measured relative to the third data packetreceived by the packet receiving unit 410, among the data packetstransmitted from the transmitting device.

According to an exemplary embodiment of the present invention, thedeviation “V_(S)” of the mean value “T_(S)” can be calculated throughEquation 6:V _(S) g*V _(S−1) +h*|V _(S−1) −T _(S)|  [Equation 6]

In this equation, “V_(S−1)” refers to the deviation calculated prior tocalculating the deviation “V_(S)”, and “T_(S)” is the mean valuecalculated through Equation 5.

In addition, “g” refers to the weight of “V_(S−1)” and “h” refers to theweight of “T_(S),” both of which follow the relation: “g+h=1.” It ispreferable that the weight “g” has a larger value than the weight “h.”Through this equation, rapid changes of the variation due to the meanvalue can be prevented, thereby obtaining a stable variation.

The initial value of the variation “V_(S)” may be set to a real numbertimes the initial value of the mean value “T_(S).”, and it is preferablethat the real number be between 0 and 1; more preferably, the realnumber may be 0.1. However, this is merely for illustrative purposes,and the initial value of the variation “V_(S)” may vary according to thewireless network environment.

The calculation unit 440 may set to an initial value of the second timezone using the initial value of the mean value “T_(S)” and the initialvalue of the deviation “V_(S).” Accordingly, when the determination unit435 determines that the packet inter-arrival time measured relative tothe newly received data packet is included in the second time zone, thecalculation unit 440 newly sets the second time zone, reflecting thepacket inter-arrival time t_(S) measured relative to the newly receiveddata packet.

The control unit 445 controls the acknowledgement packet generating unit450 and the packet transmitting unit 455 so as to transmit anacknowledgement packet corresponding to the data packet, which is freeof errors according to an assessment by the error detecting unit 415.The determination unit 435 may determine that a transmission delay hasbeen generated in the current wireless network when the packetinter-arrival time exceeds the first threshold time. If so, the controlunit 445 determines whether it is necessary to transmit anacknowledgement packet corresponding to the data packet subsequentlyreceived. The process of determining whether to transmit anacknowledgement packet will be described in detail with reference toFIGS. 5 to 11.

The acknowledgement packet generating unit 450 generates anacknowledgement packet corresponding to the data packet received by thepacket receiving unit 410, according to control by the control unit 445.

The packet transmitting unit 455 wirelessly transmits theacknowledgement packet generated by the acknowledgement packetgenerating unit 450. In the illustrated example, the packet receivingunit 410 and the packet transmitting unit 455 are present as separatemodules, but this is merely for illustrative purposes. According toanother exemplary embodiment, the packet receiving unit 410 and thepacket transmitting unit 455 may be constructed in a single integratedmodule.

Hereinafter, a process of operating a communication device according toan exemplary embodiment of the present invention will be described inmore detail with reference to FIGS. 5 to 11. For this, it is assumedthat no error has been detected in the received data packet.

FIG. 5 is a flow chart illustrating a process of operating thecommunication device according to an exemplary embodiment of the presentinvention.

When the packet receiving unit 410 receives a data packet (S110), theinter-arrival time measuring unit 430 measures the packet inter-arrivaltime of the received data packet (S120).

Thereafter, the determination unit 435 compares the packet inter-arrivaltime measured by the inter-arrival time measuring unit 430 with thefirst threshold time (S130). The first threshold time is preferably theupper limit value of the first time zone.

As a result of comparison, when the packet inter-arrival time is equalto the first threshold time or less than the first threshold time, thedetermination unit 435 compares the packet inter-arrival time with theupper limit value of the second time zone (S140). When the packetinter-arrival time is in excess of the upper limit value of the secondtime zone, the determination unit 435 determines that the packetinter-arrival time is included in the first time zone. At this time, thecalculation unit 440 resets the first time zone, reflecting the packetinter-arrival time measured in operation S120 (S150). More specifically,the calculation unit 440 calculates the upper limit value of a new firsttime zone; for this calculation, Equations 1 to 3 may be used.

As a result of the comparison in operation S140, when the packetinter-arrival time is equal to the upper limit value of the second timezone or less than the upper limit value of the second time zone, thedetermination unit 435 determines that the packet inter-arrival time isincluded in the second time zone. At this time, the calculation unit 440resets the second time zone, reflecting the packet inter-arrival timemeasured in operation S120 (S160). More specifically, the calculationunit 440 may use Equations 4 to 6 to calculate the upper limit value ofa new second time zone.

The control unit 445 controls transmission of an acknowledgement packetcorresponding to the received data packet (S170). At this time, theacknowledgement packet generating unit 450 generates an acknowledgementpacket corresponding to the received data packet and the packettransmitting unit 455 transmits the generated acknowledgement packet,according to control by the control unit 445.

Operations S110 to S170 may be repeated until transmission of the datapackets from the transmitting device is completed, as long as the packetinter-arrival time does not exceed the first threshold time.

A comparison is performed in operation S130 and if the result shows thatthe packet inter-arrival time exceeds the first threshold time, thecontrol unit 445 controls transmission of an acknowledgement packetcorresponding to the received data packet (S180). At this time, theacknowledgement packet generating unit 450 generates an acknowledgementpacket corresponding to the received data packet and the packettransmitting unit 455 transmits the generated acknowledgement packet,according to control by the control unit 445.

When the packet inter-arrival time exceeds the first threshold time, itcan be determined that a transmission delay has been generated in thewireless network. Accordingly, after having transmitted anacknowledgement packet corresponding to the received data packet at apacket inter-arrival time in excess of the first threshold time (S180),the control unit 445 limits transmission of acknowledgement packetsaccording to predetermined conditions in order to reduce unnecessaryretransmission by the transmitting device. The processes after operationS180 will be described with reference to FIGS. 6 and 7.

FIG. 6 is a flow chart illustrating a method of preventing unnecessaryretransmission in the wireless network environment according to anexemplary embodiment of the present invention.

After having transmitted an acknowledgement packet corresponding to thedata packet received at a packet inter-arrival time in excess of thefirst threshold time (S180), when a data packet is received by thepacket receiving unit (S210), the control unit 445 determines whetherthe received data packet is a previously received data packet(hereinafter referred to as “a new data packet”) (S215). When thereceived data packet is a new data packet, the control unit 445 suspendstransmission of an acknowledgement packet corresponding to the receiveddata packet (S220). Accordingly, the acknowledgement packet generatingunit 450 may not generate an acknowledgement packet corresponding to thenew data packet.

The control unit 445 also determines whether the second threshold timehas passed after having transmitted an acknowledgement packet inoperation S180 (S230). The second threshold time is a packet round-triptime estimated by the receiving device. Desirably, the second thresholdtime has the same value as the first threshold time. When the secondthreshold time has not passed since the acknowledgement packet wastransmitted in operation S180, the control unit 445 suspendstransmission of the acknowledgement packet corresponding to the new datapacket although the new data packet has been consecutively received.

When the data packet received before the second threshold time haspassed since the response packet was transmitted in operation S180,which refers to a previously received data packet (hereinafter referredto as “a duplicate data packet”), the control unit 445 controlstransmission of an acknowledgement packet corresponding to the datapacket having the highest sequence number, among the data packetsreceived after having transmitted the acknowledgement packet inoperation S180 (S225). At this time, the acknowledgement packetgenerating unit 450 generates an acknowledgement packet corresponding tothe data packet according to control of the control unit 445, and thepacket transmitting unit 455 transmits the generated acknowledgementpacket. The processes following the acknowledgement packet transmissionin operation S225 will be described with reference to FIG. 7.

In the meantime, when the second threshold time has passed under thecondition that no duplicate data packet was received after theacknowledgement packet was transmitted in operation S180, as a result ofthe determination in operation S230, the control unit 445 controlstransmission of an acknowledgement packet corresponding to the datapacket having the highest sequence number, among the data packetsreceived before the second threshold time has passed (S235).Accordingly, the acknowledgement packet generating unit 450 generates anacknowledgement packet corresponding to the data packet according tocontrol of the control unit 445, and the packet transmitting unit 455transmits the generated acknowledgement packet. At this time, thetransmitted acknowledgement packet serves to prevent generation of aretransmission timeout by the transmitting device.

Then, when the packet receiving unit 410 receives a data packet (S240),the control unit 445 determines whether the received data packet is anew data packet (S245). When a new data packet has been received, thecontrol unit 445 controls transmission of an acknowledgement packetcorresponding to the received data packet (S250).

As a result of determination in step S245, when the received data packetis a duplicate data packet, the control unit 445 controls transmissionof an acknowledgement packet corresponding to the duplicate data packet(a duplicate acknowledgement packet) (S255). Processes aftertransmission of the acknowledgement packet in operation S225 ortransmission of the duplicate acknowledgement packet in operation S255will be described with reference to FIG. 7.

FIG. 7 is a flow chart illustrating a method of preventing spuriousretransmission in the wireless network environment according to anotherexemplary embodiment of the present invention.

When a data packet is received after transmission of a response packetin operation S225 or transmission of the duplicate acknowledgementpacket in operation S255 (S310), the control unit 445 determines whetherthe received data packet is a duplicate data packet (S320). When thereceived data packet is a duplicate data packet, the control unit 445suspends transmission of a acknowledgement packet corresponding to thereceived data packet (S330). In this case, the acknowledgement packetgenerating unit 450 may not generate an acknowledgement packetcorresponding to the received data packet.

Due to suspension of the acknowledgement packet transmission, thecontrol unit 445 determines whether a third threshold time has passedunder the condition that no new data packet was received aftertransmission of an acknowledgement packet at operation S225 ortransmission of a duplicate acknowledgement packet at operation S255(S330). When the third threshold time has not passed, the control unit445 continuously suspends transmission of the acknowledgement packetcorresponding to the received data packet. In this case, operation S310to S330 are repeated. Here, the third threshold time refers to a packetround-trip time that can be estimated by the receiving device.Desirably, the third threshold time has the same value as the firstthreshold time.

However, when the third threshold time has passed under the conditionthat no new data packet was received after transmission of theacknowledgement packet at operation S225 or the duplicateacknowledgement packet at operation S255, the control unit 445 transmitsan acknowledgement packet corresponding to the data packet having thehighest sequence number, among the received data packets (S370); thisacknowledgement packet is a duplicate acknowledgement packet, serving toprevent generation of a retransmission timeout by the transmittingdevice. Thereafter, the communication device repeats processes afteroperation S110.

When a new data packet was received before the third threshold timepassed after the acknowledgement packet was transmitted in operationS225 or the duplicate acknowledgement packet in operation S255 (S320),the control unit 445 controls transmission of an acknowledgement packetcorresponding to the received data packet (S350). At this time, theacknowledgement packet generating unit 450 generates an acknowledgementpacket corresponding to a data packet as directed by the control unit445, and the packet transmitting unit 455 transmits the generatedacknowledgement packet. Thereafter, the communication device repeatsprocesses after operation S110.

In an exemplary embodiment of the present invention described withrespect to FIGS. 6 and 7, the inter-arrival time measuring unit 430measures packet inter-arrival times whenever data packets are receivedby the packet receiving unit 410 at operations S210, S240 and S310. Thedetermination unit 435 compares the measured packet inter-arrival timeswith the upper limit value of the first threshold time or the secondthreshold time, and the calculation unit 440 resets the first thresholdperiod or the second threshold period. Accordingly, the communicationdevice according to exemplary embodiments of the present inventionconducts operations S120 to S170 of FIG. 3 whenever the data packets arereceived in operations S210, S240 and S310; steps which are omitted inFIGS. 6 and 7. When a measured packet inter-arrival time exceeds thefirst threshold time while the communication device is in operation, asillustrated in the flow charts of FIGS. 6 and 7, the communicationdevice returns to operation S180 of FIG. 5 and repeats the operationsfollowing operation S180.

A packet transmitting process according to an exemplary embodiment ofthe present invention will be described with reference to FIGS. 8 to 11.In these figures, “N” to “N+10” refer to the sequence numbers of datapackets.

FIG. 8 illustrates a process of transmitting packets betweencommunication devices according to an exemplary embodiment of thepresent invention.

As illustrated, the receiving device transmits an acknowledgement packetcorresponding to a received data packet.

The transmitting device that received the response packet correspondingto the data packet “N” consecutively transmits data packets “N+1” to“N+7,” but the transmitted data packets suffer from transmission delaysin the wireless network 510. The delayed data packets may be stored inthe queue of the relaying device, which relays packet between thetransmitting device and the receiving device.

A retransmission timeout is generated 520 in the transmitting devicethat has not received a response packet within the predetermined timedue to the transmission delay. Once the retransmission timeout isgenerated, the transmitting device determines that the data packets“N+1” to “N+7” have been lost and attempts retransmission thereof. Atthis time, the TCP of the transmitting device decreases the CWND size toone TCP segment, and thus, the transmitting device preferentiallyretransmits the data packet “N+1.”

In the meantime, if the transmission delay is released within thepredetermined time due to a change in the wireless environment 530, thedata packets “N+1” to “N+7” are successfully transmitted to thereceiving device. In this case, the retransmission timeout generated bythe transmitting device is a spurious timeout.

Whenever a data packet is received, the receiving device measures apacket inter-arrival time. When the packet inter-arrival time of thedata packet “N+1” exceeds the first threshold time 540, the receivingdevice may determine that a transmission delay has been generated in thewireless network.

The receiving device transmits an acknowledgement packet correspondingto the data packet “N+1.” At this time, the transmitted acknowledgementpacket serves to prevent generation of the second retransmission timeoutby the transmitting device. At this time, transmission of theacknowledgement packet corresponds to operation S180 of FIG. 5.Accordingly, the following description of FIG. 8 is an exemplaryembodiment of operations S210 to S230 of FIG. 6.

Referring to the description with reference to FIG. 6, when a datapacket received before the second threshold time has passed after havingtransmitted the acknowledgement packet corresponding to the data packetin excess of the first threshold time (S180) is a new data packet, thecommunication device suspends transmission of an acknowledgement packetcorresponding to the received data packet (S220). Accordingly, thereceiving device in the embodiment of FIG. 8 suspends transmission ofacknowledgement packets to the data packets “N+2” to “N+7” receivedbefore the second threshold time 560 has passed after having transmittedan acknowledgement packet corresponding to the data packet “N+1” 550.

Upon receiving the acknowledgement packet corresponding to the datapacket “N+1,” the transmitting device increases the CWND size twice andtransmits the data packets “N+2” and “N+3” simultaneously. At this time,the retransmitted data packets “N+1” to “N+3” are on standby in thequeue of the relaying device while the previously transmitted datapackets “N+1” to “N+7” are being relayed. For this reason, theretransmitted data packets may also be a little delayed in transmission,as shown.

As described with respect to operations S215 and S225 of FIG. 6, when aduplicate data packet is received before the second threshold time haspassed, the communication device transmits an acknowledgement packetcorresponding to a data packet having the highest sequence number, amongthe data packets received to date (S225). In FIG. 8, since the datapacket “N+1” was re-received before the second threshold time 560passed, the receiving device transmits an acknowledgement packet 570 tothe data packet “N+7” having the highest sequence number, among thereceived data packets.

As the transmitting device has received the acknowledgement packetcorresponding to the data packet “N+7” before a retransmission timeoutis generated, it transmits data packets “N+8, N+9, . . . ” following thedata packet “N+7.”

In this exemplary embodiment, unnecessary retransmission by thetransmitting device of the data packets “N+4” to “N+7” can be prevented.

An example where the second threshold time has passed under thecondition that no duplicate data packet was received (operations S235 toS255 of FIG. 6) will be described with reference to FIG. 9.

FIG. 9 illustrates a process of transmitting packets betweencommunication devices according to another exemplary embodiment of thepresent invention.

Referring to this figure, the receiving device suspends transmission ofan acknowledgement packet corresponding to a new data packet receivedbefore the second threshold time 620 has passed after having transmittedan acknowledgement packet corresponding to the data packet “N+1” thatwas received at a packet inter-arrival time in excess of the firstthreshold time 610. In the case where the second threshold time 620passes and no duplicate data packet has been received, the receivingdevice transmits an acknowledgement packet corresponding to the datapacket “N+6” having the highest sequence number, among the data packetsreceived before the second threshold time 620 has passed. At this time,transmission of the acknowledgement packet corresponds to operation S235of FIG. 6.

Since the data packet “N+7” received thereafter is a new data packet,the receiving device transmits an acknowledgement packet correspondingto the data packet “N+7”; transmission of this acknowledgement packetcorresponds to operation S250 of FIG. 6.

However, after having received the data packet “N+7,” the receivingdevice receives a data packet “N+1” retransmitted by the transmittingdevice; that is, a duplicate data packet. Thus, the receiving devicetransmits a duplicate acknowledgement packet 630; this transmissioncorresponds to operation S255 of FIG. 6.

Meanwhile, the transmitting device that received an acknowledgementpacket corresponding to the data packet “N+6” transmits data packets“N+7”, “N+8”, and others following data packet “N+6.” According to thisembodiment, unnecessary retransmission of the data packets “N+4” to“N+6” can be prevented.

The receiving device controls transmission of the acknowledgement packetas described with respect to FIG. 7 after having transmitted anacknowledgement packet 570 and upon receiving a duplicate data packet inFIG. 8 or having transmitted a duplicate acknowledgement packet 630 inFIG. 9. Exemplary embodiments thereof are illustrated in FIGS. 10 and11.

FIG. 10 illustrates a process of transmitting packets betweencommunication devices according to another exemplary embodiment of thepresent invention.

FIG. 10 specifically depicts processes after the receiving device hasreceived the duplicate response packet 630 of FIG. 9, but processesafter the receiving device has transmitted a response packet 570 to aduplicate data packet “N+1” in FIG. 8 can be understood in the samemanner.

As described above with reference to FIG. 7, the communication devicesuspends transmission of an acknowledgement packet corresponding to adata packet received before the third threshold time passes or a newdata packet is received after having transmitted an acknowledgementpacket in operation S225 or transmitted a duplicate acknowledgementpacket in operation S255. Accordingly, the receiving device in FIG. 10suspends transmission of acknowledgement packets although duplicate datapackets “N+2, N+3 and N+7”, received after having transmitted anacknowledgement packet corresponding to a duplicate data packet “N+1”,have been received 710. When a new data packet “N+8” is received beforethe third threshold time 720 has passed, the receiving device transmitsan acknowledgement packet 730 to the new data packet. At this time,transmission of the acknowledgement packet corresponds to operation S340of FIG. 7.

Thereafter, the receiving device transmits an acknowledgement packetcorresponding to the transmitted data packet. In this exemplaryembodiment, since generation of a duplicate acknowledgement packet issuppressed, the transmitting device's generation of a spuriousretransmission can be prevented.

An example of when the third threshold time has passed before a new datapacket is received is illustrated in FIG. 11.

In this figure, the third threshold time 820 passes before a new datapacket “N+8” is received. The receiving device in FIG. 11 suspendstransmission of an acknowledgment packet although duplicate packets“N+2, N+3”, received after having transmitted an acknowledgement packetcorresponding to a duplicate packet “N+1”, have been received 810.Accordingly, the receiving device transmits an acknowledgement packet830 to the data packet “N+7” having the highest sequence number, amongthe data packets received before the third threshold time has passed.This is a duplicate acknowledgement packet 830, and transmission of theacknowledgement packet at this time corresponds to operation S360 ofFIG. 7.

Thereafter, the receiving device transmits an acknowledgement packetcorresponding to the received data packet. In this exemplary embodiment,generation of a spurious retransmission by the transmitting device isprevented by suppressing generation of the duplicate acknowledgementpackets.

As described above, a method for prevention of unnecessaryretransmission due to transmission delays in the wireless networkenvironment and a communication device using the same produce at leastone of the following effects:

First, unnecessary retransmissions due to delays in transmission of datapackets are reduced in the wireless network environment.

Second, as the unnecessary transmissions are reduced, spuriousretransmissions are accordingly prevented.

Third, as the spurious retransmissions are prevented, a phenomenon wherethe transmission rate is unnecessarily reduced is prevented.

It will be understood by those of ordinary skill in the art that variousreplacements, modifications and changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention as defined by the following claims. Therefore, it is to beappreciated that the above described embodiments are for purposes ofillustration only and are not to be construed as a limitation of theinvention.

1. A method for reducing unnecessary retransmissions due to transmissiondelays in a wireless network environment, the method comprising:measuring a packet inter-arrival time with respect to a received datapacket; transmitting an acknowledgement packet corresponding to the datapacket received at the packet inter-arrival time in excess of a firstthreshold time if the measured packet inter-arrival time exceeds thefirst threshold time; and suspending transmission of an acknowledgementpacket corresponding to a new data packet if a data packet receivedafter having transmitted the acknowledgement packet is the new datapacket that has not previously been received.
 2. The method of claim 1,further comprising, if a second threshold time has passed since havingtransmitted the acknowledgement packet, transmitting an acknowledgementpacket corresponding to a data packet having a highest sequence number,among data packets received before the second threshold time has passed.3. The method of claim 2, wherein the second threshold time has a samevalue as the first threshold time.
 4. The method of claim 1, furthercomprising, if a data packet received after having transmitted theacknowledgement packet is a duplicate data packet of the previouslyreceived data packet, transmitting an acknowledgement packetcorresponding to the data packet having a highest sequence number, amongthe data packets received before the duplicate data packet is received.5. The method of claim 4, further comprising, if a data packet receivedafter having transmitted the acknowledgement packet corresponding to thedata packet having the highest sequence number is another duplicate datapacket, suspending transmission of an acknowledgement packetcorresponding to the other duplicate data packet.
 6. The method of claim5, further comprising, if a third threshold time has passed since havingtransmitted the acknowledgement packet corresponding to the data packethaving the highest sequence number, transmitting an acknowledgementpacket corresponding to the data packet having the highest sequencenumber, among the data packets received before the third threshold timehas passed.
 7. The method of claim 6, wherein the third threshold timehas the same value as the first threshold time.
 8. The method of claim1, wherein the first threshold time refers to a value obtained byestimating a packet round-trip time of a data packet transmitted by atransmitting device.
 9. The method of claim 8, wherein the firstthreshold time is T_(L)+V_(L) where T_(L) is a mean value of packetinter-arrival times included in a first time zone, among the measuredpacket inter-arrival times, and V_(L) is a deviation of the mean valueT_(L) .
 10. The method of claim 9, wherein the first time zone is(T_(S)+V_(S), T_(L)+V_(L)), where T_(S) is a mean value of packetinter-arrival times included in a second time zone among the measuredpacket inter-arrival times, and V_(S) is a deviation of the mean valueT_(S).
 11. The method of claim 10, wherein the initial value of the meanvalue T_(L) is a measured packet inter-arrival time to the second datapacket transmitted from the transmitting device, the initial value ofthe variation V_(L) is a real number times the initial value of the meanvalue T_(L), where the real number has a positive value less than
 1. 12.The method of claim 10, wherein the second time zone is: (0,T_(S)+V_(S)).
 13. The method of claim 12, wherein an initial value ofthe mean value of T_(S) is a measured packet inter-arrival time relativeto the third data packet transmitted by the transmitting device, and aninitial value of the variation V_(S) is a real number times the initialvalue of the mean value T_(S), where the real number has a positivevalue less than
 1. 14. A communication device comprising: aninter-arrival time measuring unit which measures a packet inter-arrivaltime of a received data packet; a calculation unit which calculates afirst threshold time using the packet inter-arrival time measured by theinter-arrival time measuring unit; a determination unit which comparesthe packet inter-arrival time measured by the inter-arrival timemeasuring unit with the first threshold time calculated by thecalculation unit; and a control unit which controls transmission of anacknowledgement packet corresponding to a data packet received at apacket inter-arrival time exceeding the first threshold time if thedetermination unit determines that the packet inter-arrival time exceedsthe first threshold time, and suspends transmission of anacknowledgement packet corresponding to a new data packet if the newdata packet received after having transmitted the acknowledgement packethas not been previously received.
 15. The device of claim 14, wherein ifa second threshold time has passed since having transmitted theacknowledgement packet, the control unit controls transmission of anacknowledgement packet corresponding to a data packet having a highestsequence number, among the data packets received before the secondthreshold time has passed.
 16. The device of claim 15, wherein thesecond threshold time has a same value as the first threshold time. 17.The device of claim 14, wherein if a data packet received after havingtransmitted the acknowledgement packet is a duplicate data packet of thepreviously received data packet, the control unit transmits anacknowledgement packet corresponding to the data packet having thehighest sequence number, among the data packets received before theduplicate data packet is received.
 18. The device of claim 17, whereinif a data packet received after having transmitted the acknowledgementpacket corresponding to the data packet having the highest sequencenumber is another duplicate data packet, the control unit suspendstransmission of an acknowledgement packet corresponding to the otherduplicate data packet.
 19. The device of claim 18, wherein if a thirdthreshold time has passed since having transmitted the acknowledgementpacket corresponding to the data packet having the highest sequencenumber, the control unit transmits an acknowledgement packetcorresponding to the data packet having the highest sequence number,among the data packets received before the third threshold time haspassed.
 20. The device of claim 19, wherein the third threshold time hasthe same value as the first threshold time.
 21. The device of claim 14,wherein the first threshold time is obtained by estimating a packetround-trip time of a data packet transmitted by the transmitting device.22. The device of claim 21, wherein the first threshold time isT_(L)+V_(L), where T_(L) is a mean value of packet inter-arrival timesincluded in a first time zone, among the measured packet inter-arrivaltimes, and V_(L) is the deviation of the mean value T_(L).
 23. Thedevice of claim 22, wherein the first time zone is (T_(S)+V_(S),T_(L)+V_(L)), where T_(S) is a mean value of packet inter-arrival timesincluded in a second time zone among the measured packet inter-arrivaltimes, and V_(S) is a deviation of the mean value T_(S).
 24. The deviceof claim 23, wherein an initial value of the mean value T_(L) is ameasured packet inter-arrival time relative to the second data packettransmitted from the transmitting device, and an initial value of thevariation V_(L) is a real number times the initial value of the meanvalue T_(L), where the real number has a positive value less than
 1. 25.The device of claim 23, wherein the second time zone is: (0,T_(S)+V_(S)).
 26. The device of claim 25, wherein an initial value ofthe mean value of T_(S) is a measured packet inter-arrival time relativeto the third data packet transmitted by the transmitting device, and theinitial value of the variation V_(S) is a real number times the initialvalue of the mean value T_(S), where the real number has a positivevalue less than 1.